It's not necessary to modify your grigri, although what's done is done. I wouldn't exactly classify it as dangerous, but it certainly has that potential. Flyin' Brian McCray sailed huge on the Surgeon General (if memory serves) when his grigri didn't catch and this modification was a potential contributor.

Pull up the lead line tight and clove hitch a bomber piece every fifteen meters or so lest the rope weight pulls extra slack through-especially if you've modified. I personally almost went from the first belay into the talus off the Shortest Straw after brain farting this detail once. A guy who was soloing next to me on the first pitch of the Zodiac, 25 feet away from me when I pitched, bailed immediately.

Good rope management is absolutely key. I stack the whole rope, clip it on the back of my harness and carry it.

There's no need to be a hero rope solo free climbing, not like you need to tick the redpoint, therefore is probably wisest to mountain climb with an unmodified device i.e. hang on a piece periodically if you don't have a stance and sort out the rope/slack.